1. Field of the Invention
The invention concerns a beam designed to compensate thermal expansion of and to seal a gap between two side-by-side grate layers each made up of a longitudinal succession of alternately fixed and mobile (reciprocating) bars, said beam being made up of a longitudinal succession of elongate members with an armature fixed to a grate support frame and with two lateral rubbing members carried by said armature and urged by springs to bear against a respective grate layer edge.
2. Description of the Prior Art
Hearth grates for combustible solids, and in particular for incinerating urban waste, usually comprise a longitudinal succession of bars formed by side-by-side elements which rest with a front surface in front of a back surface of a preceding bar element within the longitudinal succession. The bars are alternately fixed and mobile (in longitudinal reciprocation) to produce a poking effect and to break up the combustible material. These structures are referred to as grates because they incorporate passages for the pressurized combustion-supporting air from underneath the grate to the combustible material. In some types of grate these orifices are formed between two side-by-side elements; in other types cf grate they are formed in the back surface of the elements.
The orifices are designed to procure a regular distribution of the combustion-supporting air over all of the grate layer, for protection against clogging by ash or unburnt material, and in some instances to orient the angle of incidence of the combustion-supporting air into the burning layer with the aim of reducing the emission of flying ash. These combustion-supporting air orifices are executed in extremely diverse ways.
Nevertheless, they all have the common feature of suffering from significant leakage of air between bar elements, which shunts the design air paths and is likely to result in an uneven distribution of the combustion-supporting air and irregular burning. The temperatures to which the grates are exposed cause them to expand and contract; it is therefore necessary to design in clearances when cold sufficient to prevent excessive loading of the hearth walls at high temperature and the risk of parts in relative movement binding.
It must be emphasized that it is not necessary to obtain a total seal, but only to limit accidental leakage to a flowrate that is low in comparison with the normal flowrate through the designed air passages, so that the grate permeability ratio, which is the ratio of the airflow cross-section to the total surface area of the grate, does not fluctuate significantly between different areas of the grate.
It is a simple matter to provide an adequate seal between bar elements in the same transverse row, as they do not move relative to each other. As a result, these elements are frequently fastened together when the grate is assembled to reduce the risk of any element being lifted by the combustible material or a foreign body introduced with the combustible material.
It is therefore at the edges of the grate layer that the risks of creating unwanted leaks are the most serious, all the more so if the clearances between side-by-side elements have been reduced, with the result that displacements due to thermal expansion accumulate at the edges. Expansion compensator seals are therefore required at the edges.
Note that a compensator seal at one edge of a layer is sufficient if the layer is not of excessive width, in which case the compensator seal at the other edge is replaced with a simple abutment surface.
It will be understood that mounting a compensator seal capable of absorbing lateral expansion clearances on a hearth wall lined with refractory bricks raises problems of fixing and of sealing, the steel or cast iron from which the compensator seal is made having a different coefficient of thermal expansion than the bricks.
In large hearths in particular, it is therefore necessary to divide the grate widthwise into layers, with compensator seals between side-by-side layers. This seal, formed by a longitudinal sequence of elongate elements, resembles a beam.
The patent FR-A-1 532 280 describes a beam of this kind, or elastically compressible separator partitions, made up of shell elements urged apart by springs or counterweights which form rubbing members bearing on the grate layer edges. In a particularly simple embodiment, the shells have a U-shaped profile and nest inside each other, sliding on a structure which is joined to the grate layer support structure.
In more complex embodiments the compensator beam includes an armature element fixed to a support structure and two shells which overlap at the top, urged apart by two crossed levers with a common central axis urged downwards by a spring or a counterweight. The effect of the crossover is, in response to a downward load on the axis, to tend to open the levers scissors-fashion and push the shells in parallel motion against edges of the layer.
The patent FR-A-2 352 250 relates to substantially the same embodiment. However, instead of sliding on the armature, the two half-shells are suspended from the ends of almost vertical interior links. This latter embodiment is directed to enabling independent movement of the two half-shells and to reducing the risk of the half-shells binding on the armature. Also, an elongate armature supports several pairs of half-shells, to enable damaged half-shells to be replaced by manipulating only parts of limited weight.
Compensator seals or beams of the prior art as described above have the disadvantage of using cast parts of relatively complicated shape and of comprising relatively complicated mechanisms which are difficult to assemble and which may not be strong enough to resist damage when foreign bodies are introduced into the hearth with the combustible material, which is not a rare occurrence in the incineration of urban waste.
Also, the stated advantages are seen to be relative in the sense that they relate to successive improvements of a common basic design.
An object of the invention is to provide a compensator seal beam as defined above which is simple in design and simple to install, having the minimum of mobile parts or articulations, and which is protected against impact from materials in the layer of combustible material.